The present invention relates to a device for measuring a chopped current, the device being of the type including a pulse transformer.
The transformer comprises a primary circuit in which the chopped current flows and a secondary circuit in which there flows an output current that is proportional to the chopped current, the secondary circuit of the transformer including two output terminals with an output current measuring circuit connected therebetween. In addition, the measuring device includes means for demagnetizing the transformer.
To a first approximation, the imperfections of a pulse transformer can be modelled by a parasitic inductance placed between the output terminals of the secondary circuit. Thus, a negligible fraction of the output current flows through this parasitic inductance. However this negligible fraction of the current, referred to as the “magnetizing current”, increases, given the potential difference maintained across the terminals of the parasitic inductance. This phenomenon referred to as the magnetization phenomenon appears as soon as the output current is measured in the measuring circuit.
Consequently, it is necessary to demagnetize the pulse transformer regularly, e.g. by making the magnetizing current flow through a demagnetizing circuit.
In general, when such a measuring device is applied to measuring a chopped current flowing in a direct current to direct current converter (DC to DC converter), the measuring circuit is constituted by a resistor connected in series with a first diode.
The demagnetizing circuit is generally constituted by a zener diode connected in series with a second diode connected in the opposite direction to the zener diode, so as to prevent any current flowing through the demagnetizing circuit while current is flowing through the measuring circuit in the direction allowed by the first diode, and conversely for allowing current to flow through the demagnetizing circuit while the first diode prevents current from flowing in the measuring circuit.
Thus, that device enables a single direction chopped current flowing in the primary circuit of the transformer to be measured. The output current induced in the secondary circuit is allowed to flow through the measuring circuit for only one of the two possible directions of current flow through the primary circuit of the transformer, the conducting direction of the first diode. This output current is proportional to the input current since the magnetizing current can be considered as being negligible. However, it cannot flow through the demagnetizing circuit, whether in a first direction because it is prevented by the second diode, or in the opposite direction because it is prevented by a positive potential difference.
Thus, the output current flows entirely through the measuring circuit and the input current ie can be deduced by measuring the output voltage Vs across the terminals of the output resistor of resistance Rs, in application of the following formula:       i    e    =                    V        s                    R        s              ×                  N        s                    N        p            where Ns is the number of turns in the secondary circuit of the pulse transformer and Np is the number of turns in the primary circuit of that transformer.
The demagnetizing operation is performed when the input current induces an output current that is zero or that is prevented from flowing through the measuring circuit because it is stopped by the first diode. The magnetizing current can thus flow freely and solely through the demagnetizing circuit, thereby enabling the energy accumulated in the parasitic inductance to be released.
The major drawback to that device is its single-directional nature. It can measure the chopped current in one direction only.
Unfortunately, it can be necessary to measure a chopped current having a mean value of zero, e.g. in order to measure the chopped current flowing in a DC-DC converter interconnecting two power-supply batteries of different voltages. Under such circumstances, the current that is to be measured necessarily includes both positive values and negative values.